I've done some sums to see what the profitability is of mining BTC and I'll post my results below.
My overclocked 5870 is working at 344 MH/s. it can go faster, but it gets too hot and the fan makes too much noise, so I dropped the speed down a notch (reducing the clock speed did not make a noticeable impact on the power use).
, 344 MH/s equates to an average time of 11 days, 21 hours, 35 minutes to mine 50 BTC. This means that my PC will mine 128 BTC a month, on average.
The current value of a BTC is approx $0.70 and the exchange rate is approx $1.60 to £1.00, so I am generating approx £56 per month.
The electric cost is 8 pence per kWh, as the expensive rate for the first x kWh is already being met by normal usage in the house, and the PC uses 300W in BTC mining mode with the monitor turned off. This means that the PC will cost £18 a month to run. In the long term I may try and build a more energy efficient mining rig, as my current PC has 2 sound cards, 3 HDDs and a quad core CPU.
I am making just over £38 a month as profit. This will obviously change as the BTC difficulty and market value fluctuate, but it may be of interest to some people.
It is worth noting that when I was mining with my HD 4870 I was actually losing £1.00 a month....